1. Field of the Invention
This invention relates to the measurement of physical values, such as absorption, reflectance, attentuance, scatterance, etc., of materials by imposing laser radiations on the material and detecting radiations emitting from such radiated material. More particularly, it concerns use of coupled-cavity lasers combined with electronic feedback circuits to provide wavelength and radiance control in the lasers as the radiation source in performing such laser radiation measurements.
2. Description of the Prior Art
Optical techniques for the measurement of a wide variety of material characteristics are extensively used in industry, research, or elsewhere to determine the physical values of materials under test. Spectrometry methods and apparatus for determination of optical absorption, attenuance, scatterance and other values of gases, liquids and solids is a typical example of such optical techniques.
Typical methods of measurement for optical absorption in transparent material are by conventional transmission, interferometry, laser intercavity absorption, photothermal detection, photoacoustic calorimetry and thermal lens calorimetry. The relative merits of these different methods have been adequately discussed in the literature (T.D. Harris, etal., Proceeding of SPIE, Vol. 426, pg. 110 T.D. Harris, Anal. Chem. 54, 1982). The present invention relates to transmission-sensitive type methods for among the full spectrum of methods available.
Such measurement operations often require highly collimated (parallel) radiation beams to attain accuracy in measurement values and propagation over long test paths. Lasers of various wavelengths and types have been used in the past in performing such measurements, e.g., study of trace materials (pollutants) in the atmosphere and elsewhere. Semiconductor (diode) lasers offer advantages in such procedures due to their small size and high conversion efficiency. However, such lasers suffer from several distinct problems, i.e., (a) emitted wavelength varies with temperature and excitation current and (b) emitted radiant flux varies substantially with emitter temperature. Methods have been proposed for correction of these defects including (1) wavelength stabilization by temperature control [L. W. Chaney, etal., Appl. Opt. 18, Sept., 17 1979], (2) wavelength stabilization by combined temperature and current control [R. A. Keller, Proc.SPIE, Vol. 426, 1983], (3) radianco stabilization by optical feedback [Amada, J. of Q.E. QE 19, Sept., 9 1983], (4) radiance stablization by synchronously modulation of beams by electro-optical feedback [Caimi, etal. Proc. SPIE Ocean Optics VII: 489, 1985], etc.
In addition to the above listed problems, well known characteristics of solid-state laser diodes predicate use of complicated bias and modulation methods to avoid facet damage and operation below threshold over temperature extremes. Although temperature control of the laser emitter is possible to eliminate these problems and provide mode stabilization, system power efficiency is compromised.
Sources of inaccuracy in measurements using prior art laser methods and devices include:
A. The laser threshold current and differential efficiency decrease with inceasing temperature and age.
B. Diode lasers can vary in wavelength while maintaining a single longitudinal mode at a bias somewhat above threshold. As temperatue increases, each longitudinal mode shifts to longer wavelengths as a result of refractive index changes.
C. Asymetric aging of front and rear facets can cause long term output radiance changes in systems deriving radiance feedback from the alternate facet.
D. The near field radiation pattern can become spotty with age. Angular changes in the far-field may result.
E. Transverse/lateral mode changes can result depending upon device structure, temperature and current.
F. Bandgap temperature dependence in any photodetector results in responsivity changes to the detected energy.
The present invention makes possible the mitigation of these problems in the optical measurement operations to which the invention is directed.
The recent development of coupled-cavity or distribuited lasers [Tsang, et al. "Semiconductors and Semimetals", Ch. 4, Vol. 22, Academic Press, 1985[ presents some advantage over the previous work cited above since very fine wavelength tuning is possible. Such coupled-cavity lasers were developed for communication systems, but in accordance with the present invention are applied with added improvements to spectrometry and comparable optical measurements. In addition, a utility of this invention is the application of cavity-tuned lasers, e.q., coupled-cavity lasers, to spectrometry of either broad or narrow absorbing test species.